Problem: In math class, the girl to boy ratio is $8$ to $9$. If there are a total of $85$ students, how many girls are there?
A ratio of $8$ girls to $9$ boys means that a set of $17$ students will have $8$ girls and $9$ boys. A class of $85$ students has $5$ sets of $17$ students. Because we know that there are $8$ girls in each set of $17$ students, the class must have $5$ groups of $8$ girls each. There is a total of $40$ girls in math class.